Broadband noise jammers can destructively interfere with each other in space in a manner similar to that well-known for continuous wave jammers. This destructive interference between broadband jammers occurs with each other for time periods which are on the order of the reciprocal of their effective bandwidths. In general, the effective bandwidth of the jammers is that bandwidth which can enter a radar receiver (i.e., the radar bandwidth). This destructive interference allows two or more noise jammers located at different azimuth angles from the main radar antenna to cancel each other (e.g., at the radar antenna) for short periods of time, and simultaneously to not cancel each other at one or more auxiliary antennas at a different location from the main radar antenna. Cancellation, or destructive interference can occur (at the radar antenna) between two or more jamming signals while constructive interference between the jamming signals at one or more of the auxiliary antennas is simultaneously present.
During periods of cancellation between interfering broadband noise signals, limited bandwidth loops, generally important elements of sidelobe cancellers, cannot respond quickly enough to a sudden reduction of the jamming signal (i.e., "dropout") in the received radar signal. (A typical signal canceller employing loops of this nature is described in U.S. Pat. No. 3,202,990 to P. W. Howells.) Because the canceller loops cannot respond quickly enough, they continue to subtract (from the radar signal) a signal proportional to the strength of the jamming signal being received. Since the jamming signal simultaneously being received at the auxiliary antenna is not correlated with any signal from the main radar antenna, the subtraction itself has the same effect as if the main radar signal were being interfered with by a uncancelled jammer signal.
The present invention provides a technique for utilizing the unmodified radar signal during periods when the jammer signals are reduced or cancelled at the radar antenna.